Magnetic recording tape is subject to wear as it is directed by stationary guide means such as are used in many tape transport mechanisms. The problem is intensified when processing video signals because of the extremely high tape speeds involved and, consequently, the multiplicity of tracks which must be used to record or playback a program of convenient length in a manageable length of tape. For example, in a system which operates at a tape speed of 120 inches per second, 36,000 feet of tape pass the transducing head each hour. Due to physical limitations in reel size, it is well known to record the data on a multiplicity of parallel tracks and to repeatedly pass the shorter length of tape past the transducer, each time reading information from a different track. In this manner a 1200 foot reel of tape having 30 tracks can be used to record or playback a 60 minute program on the above-described system. The requirement that the tape pass through the transport 30 times per program, however, results in the serious wear considerations mentioned above.
Guides utilizing a film of air as a lubricant are well known in the art. Known guides, however, have serious disadvantages.
One type of known guide has a guiding periphery which is linear across the width of the tape and has openings along the periphery through which compressed air is passed to create a lubricating film of air. Typically single ports placed along the guide periphery at the centerline of the tape path are used. Such configuration makes it likely that an unequal pressure distribution across the width of the tape will be generated. Given the linear configuration of the guide, the higher pressure at the centerline of the tape will cause the tape to "bulge." Since, by their nature, tape guides direct the tape through a curved path, the "bulging" causes undesirable deformation of the tape as a result of the requirement that it flex in two planes simultaneously. As a related consequence the tape in resisting the deformation tends to "squeeze" the air film out unless significant pressure is maintained at the air inlet ports. As the pressure is increased, the volume of compressed air required is also increased, due to the leakage between the tape edges and the guide. The leakage can be reduced by maintaining close tolerance between the edges of the tape and guiding flanges of the guide but this approach introduces at least two further disadvantages. Not only is the manufacturing cost of both the tape and the guide increased as a result of the tighter tolerances, but edge wear is likely to be encountered. This wear is especially undesirable when using tape having a large number of parallel tracks requiring highly accurate lateral positioning of the tape.
To overcome some of the above disadvantages, multiplicity of ports across the width of the tape having varying sizes and/or varying inlet pressures is known to produce a more uniform pressure distribution. The use of this technique, however, increases the manufacturing costs significantly.
An alternative means of producing a more uniform pressure across the width of the tape and also along the path of the tape is through the use of air pockets instead of simply ports through which compressed air is introduced between the tape and the guide. Although more uniform pressure is achieved in this manner, it has been either at the cost of significantly increased air consumption or through a relatively complex design and the associated higher manufacturing costs.